1. Field of the Invention
The present invention relates to photo responsive semiconductor devices, and in particular to photo transistors having multi-apertured collector electrodes.
2. Description of the Prior Art
The frequency response of bipolar photo-transistors is largely limited by the product of base resistance and emitter capacitance. Bipolar phototransistors have largely been designed for low speed, low power operation where a high base resistance is not a problem. Some prior art high frequency photo-transistor designs have attempted to reduce the base resistance, as by interdigitating the emitter and base, but have not been able to achieve the desired frequency response.
One method for increasing the frequency response of bipolar phototransistors involves forming apertures or openings in the collector electrode for admitting light therethrough in order to generate carriers close to the base-collector junction. Current carriers are instantly subjected to the high space-charge field surrounding the depletion region and, thus, fast turn-on can be achieved by a rapid rise-time light pulse. However, the power-frequency capabilities of the apertured collector configuration is limited by the power-frequency capabilities of available light sources. That is, for example, where high power laser diodes are used as a light source for firing an apertured collector photo-transistor, a frequency vs. power trade-off is required for the laser diode. The more power required from the laser diode, the lower the possible repetition rate or frequency. Or, conversely, the higher the frequency desired from the laser diode, the lower the possible power from the laser diode.